Good question.But ,I think it depends upon type of beam deep or not.

Normally angle of shear crack with axis of beam decreases as span to depth ratio of beam increases. For long span beam flexural effect is more than shear.

Effect of percentage of steel reinforcement also affect propagation of shear crack.

Concentrated load acing on a beam will create larger angle of shear crack and it will also depends on detailing of beam and width of beam transferring the load.

i took courses in concrete structures and i wrote two projects alone about concrete bridge and concrete foundation and i used analysis in laboratory about the tensile stress and shear force when i analysis to know the capacity of the concrete and the durability of the concrete structures .But when we come to your question my view is it depends on the function of the graph which shows the curve of the strength of materials that means the shear force and the stress we can not assume on line the angle of the shear stress and the angle of the crack

some times it is 45 degree some times it is between 1 and 15 degrees as i mentioned above we can not assume on line with out laboratory test of the materials

this summery i have got from some one please read it to have some knowledge about it " The effects of the various influential parameters, such as beam size, shear span to depth ratio, side concrete cover to stirrup, stirrup spacing, stirrup configuration and longitudinal reinforcement ratio on the spacing between shear cracks and the relationship between shear crack width and stirrup strain at the intersection with shear cracks were carefully investigated. It was found that shear crack width proportionally increases with both the strain of shear reinforcement and with the spacing between shear cracks. Greater shear crack spacing's were found in larger beams and hence resulted in wider shear crack width. The test results also revealed that shear reinforcement characteristics (side concrete cover to stirrup, stirrup spacing and/or stirrup configuration) and longitudinal reinforcement ratio play a critical role in controlling the shear crack spacing's and openings. As a result of this study, a simplified formula for the calculation of shear crack spacing as well as more accurate model for the prediction of shear crack width in reinforced and prestressed concrete beams with shear reinforcement are proposed. The simplified formula to calculate shear crack spacing represents the crack control characteristics of both the longitudinal and transverse reinforcement. The proposed model to predict shear crack width is based on the relationship between shear crack width and stirrup strain at the intersection of stirrup with shear crack taking into consideration the spacing between shear cracks and the additional influential parameters on shear crack width. Finally, spacing's and widths of shear cracks estimated by the proposed models show a better correlation (average and coefficient of variation) with the test results compared to the other existing prediction formulae. The effects of prestressing tendon can be considered in the ratio of longitudinal reinforcement and the effective concrete area surrounding longitudinal reinforcement. "